Flying spot scanners (often referred to as raster output scanners (ROS)) conventionally have a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction (also known as the fast-scan direction) while the recording medium is being advanced in an orthogonal, or "process", direction (also known as the slow scan direction) such that the beams scan the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with a binary sample stream, whereby the recording medium is exposed to the image represented by the samples as it is being scanned. Printers that sweep several beams simultaneously are referred to as multibeam printers. Both ROS and multibeam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura.
In the Kitamura patent, multiple lasers are arranged diagonally (see FIG. 10B of the Kitamura patent) to sweep multiple lines across a single photoreceptor. The beams are displaced from each other in the cross-scan direction so that multiple lines can be scanned simultaneously across the photoreceptor. In addition, an object of the Kitamura patent is to reduce variations in pitch by spacing individual lasers within the laser array closely in a compact structure.
High speed process color or multi-highlight color xerographic image output terminals require multiple independently addressable raster lines to be printed simultaneously at separate exposure stations. This is called multistation printing. Conventional architectures for multistation process color printers use a plurality of separate ROSs, usually four independent ROSs, one for each system color for example, as illustrated in U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al. the disclosures of which are herein incorporated by reference.
Problems with these systems include the high cost related to the cost of multiple ROSs, the high cost of producing nearly identical multiple ROSs and associated optics, and the difficulty of registering the system colors.
A partial solution to the problems of multistation xerographic systems with individual ROSs is disclosed in U.S. Pat. No. 4,591,903 to Kawamura et al. U.S. Pat. No. 4,591,903, particularly with regard to FIG. 6 discloses a recording apparatus (printer) having multiple recording stations and multiple lens systems, but only two polygon mirrors. With two polygon mirrors and only one associated drive motor, the cost of the system is reduced. However, differences in the lenses, the polygon and mirror surfaces could still cause problems with color registration.
Another approach to overcoming the problems of multistation printers having individual ROSs is disclosed in U.S. Pat. No. 4,962,312 to Matuura. U.S. Pat. No. 4,962,312 discloses overlapping a plurality of beams using an optical beam combiner, deflecting the overlapped beams using a single polygon mirror, separating the deflected beams using an optical filter (and polarizers if more than two beams are used), and directing the separated beams onto associated photoreceptors. The advantage of overlapping the laser beams is a significant cost reduction since the ROS (i.e., the polygon mirror and all optics) is shared by all the stations.
However, an actual embodiment of the apparatus described in U.S. Pat. No. 4,962,312 would be rather complicated and expensive, especially if four system colors are to be printed. The use of optical beam combiners to overlap beams so that they have similar optical axes would be difficult, expensive and time consuming. Obtaining similar size spots on each photoreceptor would also be difficult because it would be difficult to establish the same optical path lengths for each beam. It would also be difficult to ensure that the latent images on the photoreceptors are registered. Each of these problems is at least partially related to the relative positions of the laser sources.
U.S. Pat. No. 5,243,359 to Tibor Fisli, the disclosure of which is incorporated herein by reference in its entirety, discloses a ROS system suitable for deflecting multiple laser beams in a multistation printer. A rotating polygon mirror simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams, having, their largest divergence angles parallel to one another that are subsequently separated by a plurality of optical filters and directed onto their associated photoreceptors. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar optical path lengths for each beam. This is facilitated by locating all lasers in an integral unit. However, laser diodes illustrated in U.S. Pat. No. 5,243,359 are arranged in a line in the cross scan direction, i.e., parallel to the axis of rotation of the polygon mirror (see FIG. 2 of U.S. Pat. No. 5,243,359. Diodes oriented in the cross-section direction must be arranged such that they are packed closely in a direction parallel to the polygon mirror rotation axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. That is, the laser diodes are kept as close together as possible in the direction parallel to the polygon mirror rotation axis (i.e., the height direction of the polygon mirror) so that the light beams strike as nearly the same portion of the polygon mirror as is possible. However, since the light beams are spaced from each other in the height direction of the polygon mirror, the beams do not all strike the same portions of each facet of the polygon mirror, and therefore, are not uniformly reflected. Additionally, locating the laser diodes close to each other introduces cross-talk.